BEHAVIOR OF VAPOR BUBBLES GENERATED ON A SINGLE HEATER ROD PARTLY CONFINED BY METALLIC WALLS IN SUBCOOLED WATER

摘要

Vibration of a structure should be avoided to prevent any damage, such as fretting wear occurring between its components, leading to critical failure of the structure. As an industrial example, a fuel assembly in the reactor core of a pressurized water reactor (PWR) is considered as such a structure. A fuel assembly for a PWR, consisting of hundreds of Arm-long fuel rods, is utilized in the vertical upflow of the coolant water in a reactor core under high-pressure and high-temperature conditions. Abnormal situations, such as system pressure drop or loss of coolant flow in the core may lead to unexpected subcooled boiling at the fuel rods, which generate heat due to the nuclear reaction. The behavior of vapor bubbles generated in subcooled boiling can be a cause of fuel rod vibration, i.e., subcooled boiling-induced vibration (SBIV). This paper describes some experiments studying SBIV of a single heater rod in distilled water under atmospheric pressure in order to understand the fundamental phenomena of the SBIV. These experiments focused on the effects of the structural constraints formed by metallic walls around the heater rod on the SBIV of the rod. Three configurations of structural constraints were provided in the experiments-single, parallel, and corner walls-under the thermal conditions of constant heat flux and in a certain range of subcooling degrees. The behavior of the generated vapor bubbles was observed to depend, in each case, on the structural constraints by the walls, which influenced the measured acceleration of the SBIV of the heater rod.